1. Field of the Invention
The present invention relates to a thermal dissipation device having improved surface area and fluid flow characteristics resulting in high thermal transfer efficiency.
2. Description of the Related Art
Thermal dissipation devices are present in a wide variety of applications, including electronic apparatus such as computers, stereos, televisions, or any other device that produces unwanted heat by inefficiencies in electronic circuits, such as integrated circuit chips (ICs), including microprocessors.
Among the factors that influence the design of a thermal dissipation device are the principles that: 1) increasing surface area of the thermal dissipation device generally improves thermal transfer, and 2) increasing fluid flow over the device generally improves thermal transfer. A heat sink is a thermal dissipation device, typically comprising a mass of material (generally metal) that is thermally coupled to a heat source and draws heat energy away from the heat source by conduction of the energy from a high-temperature region to a low-temperature region of the metal. The heat energy can then be dissipated from a surface of the heat sink to the atmosphere primarily by convection. A well known technique of improving the efficiency of a conductive heat sink is to provide a greater surface area on the heat sink, typically provided by fins that are formed on a base portion of the heat sink, so that more heat can dissipate from the heat sink into the atmosphere by natural (or free) convection. The thermal efficiency of a heat sink can be further increased by employing forced convection wherein a flow or stream of fluid, typically a gas such as air, is forced over and around the surface of the heat sink.
Current heat sinks increase surface area by including a number of raised, rectangular cross-section beams, or fins. If a heat source produces enough heat that forced convection is required to maintain the heat source within an appropriate operating temperature range, a fan is mounted to provide air flow over the fins to dissipate a greater amount of heat energy. For purposes of explanation, the heat source described herein is an integrated circuit (IC). However, it should be understood that the heat source may be any device that generates heat.
Some thermal dissipation devices use rod-shaped pins ("pin fins"), as illustrated in the cross sectional side view in FIG. 1. Pin fins 102 are in thermal contact with and extend from the top of base 101 of heatsink 100. The pins may be integrally formed or later affixed to the base 101. Each pin has a diameter D, an overall length L, and if applicable, a depth B of insertion into the base 101. While the pins are illustrated as being of circular cross-section, any suitable cross section may be employed, with the understanding that a smooth, circular cross section minimizes air flow resistance, while rough, square, complex (e.g., star shaped) or irregular cross section will increase airflow resistance and surface area available for convection.
The base, or plate, of the heat sink device may have a flat surface or curved surface in different embodiments. The bottom surface of base 101 generally is coupled directly, or indirectly, to the IC to dissipate heat from the IC. The heat travels through the heat sink base 101 and then through pins 102 by conduction. At the top surface of base 101 and the surface of pins 102, the heat is dissipated into the atmosphere by natural or forced convection. A fan commonly is utilized to generate additional airflow across heat sink 100 to dissipate a greater amount of heat energy. FIG. 2 provides a top view in which a number of pins rise from base 101, spaced and aligned to form a grid on the top surface of the base 101 of heat sink 100.
Presently, pin fins are limited by a relatively low length:width ratio. Reasonably inexpensive pin fins generally are limited to a length:width ratio of approximately 8:1, in part due to their being fabricated by casting. More expensive pin fins might reach a length:width ratio as high as 15:1. Due to limitations of known manufacturing methods, there is a trade off between length:width ratio and occupancy ratio.
With reference to FIG. 3, occupancy ratio is measured as the percentage of surface area of the body of a heat sink that is occupied by the cumulative cross sectional area of the pin fins. In the case of a square or rectangular area on the surface of the heat sink, the pins, of radius R, are arranged in rows on dimension X centers and in columns on dimension Y centers. The combination of pins in rows and columns forms a grid pattern. In this case, the occupancy of the overall grid is measured by taking the occupancy of one X-by-Y area: EQU overall area=XY EQU rod area=.pi.R.sup.2
and thus occupancy ratio is .pi.R.sup.2 /XY. In the case of a square grid, where X and Y are equal and the rows and columns are at right angles, occupancy can be stated more simply as an occupancy ratio .pi.D/4X where D is the diameter of the pins and X is the on-center distance between the pins. Given small geometries and large pin heights in relation thereto, existing pin fin architectures are limited to a fairly low occupancy ratio, principally governed by existing manufacturing methods. Prior thermal dissipation systems rely on natural, or forced convection generated by a fan or other inefficient air flow device. The heat sinks employ fins and generally mount the fan or blower adjacent or above the heat sink fins. In these and other prior art systems, the challenge is generating sufficient airflow past a maximum amount of surface area of the heat sink, while minimizing manufacturing cost and space requirements.